Nitride semiconductor laser device

ABSTRACT

A nitride semiconductor laser device of high reliability such that the width of contact between a p-side ohmic electrode and a p-type contact layer is precisely controlled. The device comprises a substrate, an n-type nitride semiconductor layer, an active layer, and a p-type nitride semiconductor layer. All the layers are formed in order on the substrate. A ridge part including the uppermost layer of the p-type nitride semiconductor layer of the p-type nitride semiconductor layer i.e., a p-type contact layer is formed in the p-type nitride semiconductor layer. A p-side ohmic electrode is formed on the p-type contact layer of the top of the ridge part. A first insulating film having an opening over the top of the ridge part covers the side of the ridge part and the portion near the side of the ridge part. The p-side ohmic electrode is in contact with the p-type contact layer through the opening. A second insulating film is formed on the first insulating film.

TECHNICAL FIELD

[0001] The present invention relates to a laser device made of a nitridesemiconductor (Al_(b)In_(c)Ga_(1-b-c)N, 0≦b, 0≦c, b+c<1).

BACKGROUND ART

[0002] Recently, nitride semiconductor laser devices capable ofoscillating in blue wavelength region have been receiving muchattention. Also there have recently been demands for a nitridesemiconductor laser device that has a high output power. To meet thesedemands, a laser having ridge structure formed by partially etching ap-type contact layer and a part of a p-type cladding layer has beenstudied so as to achieve single transverse oscillation mode having goodFFP (far field pattern) However, the ridge has a very small width, from1 μm to 2 μm, and it is difficult to form an ohmic electrode having thesame width as that of this ridge. To circumvent this difficulty, ap-type ohmic electrode that contacts with a nitride semiconductor onlyon the top surface of the ridge has been formed, by covering the entirenitride semiconductor except for the end face with an insulation filmwith only an n-electrode forming surface being left exposed, thenforming the electrode that has a roughly determined width on the ridge.Then a pad electrode of 2-layer structure is formed on the p-type ohmicelectrode by forming an Au film on an Au or Ni film.

[0003] With the nitride semiconductor laser device of the prior art,however, since the insulation film that covers the entire nitridesemiconductor except for the end face needs to have a thickness not lessthan a certain value in order to protect the device, there has been sucha problem that an aperture cannot be precisely formed on the top surfaceof the ridge, and therefore it is difficult to precisely control thewidth of contact between the p-type ohmic electrode and the p-typecontact layer. Thus it has been difficult to manufacture a laser devicewith minimum variations in the device characteristics.

[0004] Also there has been such a problem that heat generated when thelaser device is powered causes Au atoms of the pad electrode togradually diffuse into a layer underneath, resulting in deterioration ofthe device characteristics. Particularly when mounting a laser deviceface-down, conspicuous deterioration of the device characteristicsoccurs since the diffusion of Au is accelerated by heating to atemperature of about 350° C. for the purpose of face-down mount.

DISCLOSURE OF THE INVENTION

[0005] An object of the present invention is to provide a nitridesemiconductor laser device that has high reliability with lessdeterioration of the device characteristics, where the width of contactportion between a p-type ohmic electrode and a p-type contact layer canbe precisely controlled.

[0006] In order to achieve the object described above, the nitridesemiconductor laser device of the present invention comprises an n-typenitride semiconductor layer, an active layer and a p-type nitridesemiconductor layer, that are formed successively on a substrate, with aridge that includes at least a p-type contact layer in the top layerbeing formed in the p-type nitride semiconductor layer, and a p-typeohmic electrode that makes ohmic contact with the p-type contact layerformed on the ridge being formed substantially parallel to the directionof resonance, wherein a first insulation film that has an apertureformed at a position over the ridge is formed so as to cover at leastthe side face a proximate region outside the side face of the ridge, thep-type ohmic electrode is formed so as to make contact with the p-typecontact layer through the aperture, and a second insulation film isformed on the first insulation film.

[0007] The constitution described above makes it possible to preciselycontrol the width of contact between the p-type ohmic electrode and thep-type contact layer by forming the first insulation film precisely, andeffectively protect the device by means of the second insulation filmformed on the first insulation film, thereby achieving the nitridesemiconductor laser device that has stable characteristics and highreliability.

[0008] In the nitride semiconductor laser device of the presentinvention, the second insulation film may be formed in continuation witha resonating end face so as to form a laser reflection plane on theresonating end face.

[0009] This constitution enables it to form the second insulation filmand the laser reflection plane in a single process.

[0010] In the nitride semiconductor laser device of the presentinvention, the first insulation film and the second insulation film arepreferably formed from oxide compound.

[0011] In the nitride semiconductor laser device of the presentinvention, the first insulation film is preferably formed from ZrO₂.

[0012] In the nitride semiconductor laser device of the presentinvention, the second insulation film is preferably formed from TiO₂ orSiO₂. Use of these materials enables the second insulation film to beformed in continuation with the resonating end face so as to form thelaser reflection plane on the resonating end face, so that the secondinsulation film and the laser reflection plane can be formed in a singleprocess.

[0013] When the second insulation film is formed in continuation withthe resonating end face so as to form the laser reflection plane on theresonating end face, it is more preferable that the second insulationfilm is a multi-layered film made by forming TiO₂ layer and SiO₂ layerone on another.

[0014] In the nitride semiconductor laser device of the presentinvention, the p-type ohmic electrode is preferably made of an alloy byforming a layer of at least one kind selected from among a groupconsisting of Ni, Co, Fe, Ti and Cu and an Au layer one on another, andthen annealing the layers.

[0015] In the nitride semiconductor laser device of the presentinvention, such a process may be employed as the second insulation filmis formed so as to have an aperture located over the p-type ohmicelectrode and the p-type pad electrode is formed so as to make contactwith the p-type ohmic electrode through the aperture.

[0016] In the nitride semiconductor laser device of the presentinvention, it is preferable to form the p-type pad electrode so as toinclude a bonding layer that makes contact with the p-type ohmicelectrode, a barrier layer and an Au layer that are formed in thisorder, while the bonding layer is made of a material that bonds betterwith the second insulation film and with the p-type ohmic electrode thanthe Au layer does, and the barrier layer is made of a material that isless likely to diffuse than the Au layer.

[0017] Such a constitution makes it possible to increase the bondingstrength of the p-type ohmic electrode and the p-type pad electrode, andprevent Au located at the top of the pad electrode from diffusing intothe other layers due to the heat generated by the current supplied tothe device.

[0018] In the nitride semiconductor laser device of the presentinvention, it is preferable to form the bonding layer of the p-type padelectrode so as to include at least one material selected from among agroup consisting of Ni, Cu, Ru, RuO₂, Ti W, Zr, Rh and RhO, in order tofurther increase the bonding strength of the p-type ohmic electrode andthe p-type pad electrode.

[0019] In the nitride semiconductor laser device of the presentinvention, it is preferable to form the barrier layer of the p-type padelectrode so as to include at least one material selected from among agroup consisting of Ti, Pt, W, Ta, Mo, nitride thereof and RhO, in orderto effectively prevent Au from diffusing into the other layers.

[0020] In the nitride semiconductor laser device of the presentinvention, in case the n-type nitride semiconductor layer includes then-type contact layer that is partially exposed and the n-type padelectrode is formed on the exposed n-type contact layer via the n-typeohmic electrode, the n-type pad electrode is preferably made of the samematerial as the p-type pad electrode.

[0021] This constitution enables the n-type pad electrode and the p-typepad electrode to be formed in the same process.

[0022] In the nitride semiconductor laser device of the presentinvention, such a constitution may be employed as the p-type ohmicelectrode is made of an alloy by forming a layer of at least one kindselected from among a group consisting of Ni, Co, Fe, Ti and Cu and anAu layer one on another, and then annealing the layers, wherein thesecond insulation film is formed so as to have an aperture at a positionlocated over the p-type ohmic electrode and the p-type pad electrode isformed so as to make contact with the p-type ohmic electrode through theaperture.

[0023] In the nitride semiconductor laser device of the presentinvention, the p-type pad electrode may also be constituted from thebonding layer formed from Rh or RhO in contact with the p-type ohmicelectrode, and the Au layer formed on the bonding layer.

[0024] This constitution results in improved heat resistance of thep-type ohmic electrode and the p-type pad electrode.

[0025] In this case, the p-type pad electrode may also be constitutedfrom a bonding layer that is formed from Rh or RhO and makes contactwith the p-type ohmic electrode, a barrier layer formed on the bondinglayer from a material including at least one material selected fromamong a group of Ti, Pt, W, Ta, Mo and nitride thereof, and an Au layerformed on the barrier layer.

[0026] In order to improve the heat resistance of the p-type ohmicelectrode and the p-type pad electrode further, such a constitution ispreferable as an RhO layer is included in the top layer of the p-typeohmic electrode and the bonding layer is made of RhO.

[0027] The nitride semiconductor device of the present inventioncomprises a p-type nitride semiconductor layer, a p-type ohmic electrodeformed on the p-type nitride semiconductor layer, and a p-type padelectrode formed on the p-type ohmic electrode, wherein the p-type ohmicelectrode is made of an alloy by forming a layer of at least one kindselected from among a group consisting of Ni, Co, Fe, Ti and Cu and anAu layer one on another, and then annealing the layers, while the p-typepad electrode is constituted from a bonding layer formed from Rh or RhOin contact with the p-type ohmic electrode, a barrier layer formed onthe bonding layer from at least one material selected from among a groupof Ti, Pt, W, Ta, Mo and nitride thereof, and an Au layer formed on thebarrier layer.

[0028] In the nitride semiconductor device of the present inventionhaving the constitution described above, good ohmic contact can beestablished between the p-type nitride semiconductor layer and thep-type nitride semiconductor layer, and the heat resistance of thep-type ohmic electrode and the p-type pad electrode can be improved,thereby achieving the nitride semiconductor device having a long servicelife.

[0029] In the nitride semiconductor device of the present invention, inorder to improve the heat resistance of the p-type ohmic electrode andthe p-type pad electrode further, such a constitution is preferable asan RhO layer is included in the top layer of the p-type ohmic electrodeand the bonding layer is made of RhO.

[0030] A method for forming the electrodes of the nitride semiconductordevice is a process of forming the electrodes on the p-type nitridesemiconductor layer, and comprises a step of forming the p-type ohmicelectrode by forming a first layer made of at least one kind selectedfrom among a group consisting of Ni, Co, Fe, Ti and Cu, an Au layer andan RhO layer successively on the p-type nitride semiconductor layer, astep of annealing the p-type ohmic electrode, a step of forming an RhOlayer on the p-type ohmic electrode that has been annealed and a step offorming the p-type pad electrode on the p-type ohmic electrode includingthe formation of the Au layer.

[0031] In the nitride semiconductor device manufactured by the method ofthe present invention described above, good ohmic contact can beestablished between the p-type nitride semiconductor layer and thep-type nitride semiconductor layer, and the heat resistance of thep-type ohmic electrode and the p-type pad electrode can be improved,thereby achieving the nitride semiconductor device that has a longservice life.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 is a schematic sectional view showing the constitution ofthe nitride semiconductor laser device according to an embodiment of thepresent invention.

[0033]FIG. 2 is a partial sectional view showing the constitution of thenitride semiconductor laser device according to a variation of thepresent invention.

[0034]FIG. 3 is a perspective view of the nitride semiconductor laserdevice shown in FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

[0035] Now the semiconductor laser diode according to an embodiment ofthe present invention will be described below with reference to theaccompanying drawings. FIG. 1 is a schematic sectional view showing theconstitution of a semiconductor laser diode according to thisembodiment, showing a cross section perpendicular to the direction oflaser oscillation.

[0036] The semiconductor laser diode of this embodiment is constitutedfrom a plurality of semiconductor layers comprising a buffer layer (notshown), an n-type contact layer 2, an n-type cladding layer 3, an n-typeoptical guide layer 4, an active layer 5, a p-type cap layer 6, a p-typeoptical guide layer 7, a p-type cladding layer 8 and a p-type contactlayer 9, formed successively as shown in FIG. 1, wherein a p-type ohmicelectrode 20 is formed, so as to make contact with the p-type contactlayer 9 through an aperture 30 a of a first insulation film 30, on thep-type contact layer 9 having a ridge shape that is long enough in thedirection of resonance, and an n-type ohmic electrode 21 is formed so asto make contact with the n-type contact layer 2 through an aperture 30 bof the first insulation film 30, on the n-type contact layer 2 that hasbeen exposed by etching.

[0037] The semiconductor laser diode of this embodiment is furtherprovided with a second insulation film 31 that has apertures 31 a, 31 bat positions located over the p-type ohmic electrode 20 and the n-typeohmic electrode 21, respectively, while a p-type pad electrode 22 and ann-type pad electrode 23 are formed in electrical continuity with thep-type ohmic electrode 20 and the n-type ohmic electrode 21,respectively, via the apertures 31 a, 31 b.

[0038] In the semiconductor laser diode of this embodiment, the firstinsulation film 30 is formed mainly for the purpose of putting thep-type ohmic electrode 20 in satisfactory contact with the top surfaceof the p-type contact layer 9 (to ensure the accuracy in the shape ofthe area that makes ohmic contact), and is required to have enough heatresistance since annealing is required after forming the p-type ohmicelectrode 20.

[0039] Since the first insulation film 30 is formed on both sides of theridge, refractive index of the first insulation film 30 must be lowerthan that of the p-type nitride semiconductor that constitutes the ridge(preferably as near to the dielectric constant of vacuum as possible).

[0040] Moreover, since the first insulation film 30 is formed near theridge and it is necessary to form the aperture 30 a with a highaccuracy, the first insulation film 30 must be thin. For example, sincethe ridge is normally about 1.5 μm and 0.5 μm in width and height,respectively, the thickness of the first insulation film 30 is set to0.5 μm or less.

[0041] In the semiconductor laser diode of this embodiment, the secondinsulation film 31 is formed mainly for the purpose of protecting thedevice, and is therefore made of a material that has effectiveprotective function.

[0042] In this embodiment, the second insulation film 31 has theapertures 31 a, 31 b formed therein at positions above the p-type ohmicelectrode 20 and the n-type ohmic electrode 21, respectively, while thep-type pad electrode and the n-type pad electrode are formed inelectrical continuity with the p-type ohmic electrode 20 and the n-typeohmic electrode 21, respectively, via the apertures 31 a, 31 b.

[0043] However, since the aperture 31 a used for the continuity betweenthe p-type ohmic electrode 20 and the p-type pad electrode and theaperture 31 b used for the continuity between the n-type ohmic electrode21 and the n-type pad electrode are not required to have such a highdimensional accuracy as that of the aperture 30 a of the firstinsulation film 30, the second insulation film 31 may be formed with arelatively large thickness.

[0044] Moreover, since the second insulation film 31 is not subjected toannealing after being formed as in the case of the first insulationfilm, the second insulation film 31 is not required to have as high heatresistance as the first insulation film 30.

[0045] Therefore, the second insulation film 31 may be made of amaterial that can effectively perform protective function, selected onthe basis of appropriateness for the semiconductor laser diode.

[0046] In the semiconductor laser diode of this embodiment, the p-typepad electrode 22 comprises three layers of a bonding layer 22 a thatmakes contact with the p-type ohmic electrode, a barrier layer 22 b andan Au layer 22 c.

[0047] In this embodiment, the bonding layer 22 a of the p-type padelectrode 22 is made of such a material that bonds well with the secondinsulation film 31 that is formed on the ridge and with the p-type ohmicelectrode 20, and is less likely to diffuse, for which preferablecandidates are Ni, Cu, Ru, RuO₂, Ti, W, Zr, Rh and RhO. When the secondinsulation film is made of an oxide, in particular, the most preferablematerial is Ni that has high bonding characteristic with the secondinsulation film. In order to improve the heat resistance of the p-typepad electrode 22, the bonding layer 22 a is preferably made of Rh orRhO.

[0048] The thickness of the bonding layer 22 a is preferably in a rangefrom 100 Å to 5000 Å, most from 500 Å to 2000 Å.

[0049] When the bonding layer 22 a is made of Rh or RhO, the Rh layer orthe RhO layer also functions as a barrier layer that prevents the Aulayer 22 c from diffusing. Therefore, when the bonding layer 22 a ismade of Rh or RhO, the barrier layer 22 a can be omitted so as toconstitute the p-type pad electrode 22 from two layers of the Rh layeror RhO layer and the Au layer 22 c.

[0050] When the p-type pad electrode 22 is constituted from two layersof the Rh layer or RhO layer and the Au layer 22 c, heat resistance ofthe p-type pad electrode 22 can be made equivalent to or better thanthat of the p-type pad electrode constituted from the other combinationdescribed in this embodiment.

[0051] When the p-type pad electrode 22 is constituted from two layersof the combination described above, the thickness of the Rh layer or RhOlayer is preferably in a range from 100 Å to 10000 Å, and the thicknessof the Au layer 22 c is preferably from 1000 Å to 30000 Å.

[0052] According to the present invention, the barrier layer 22 b of thep-type pad electrode 22 is made of such a high-melting point metal as Auatoms in the top layer does not diffuse into the bonding layer or thelower layers or a nitride thereof, for example, Ti, Pt, W, Ta, Mo orTiN, and most preferably Ti. The thickness is preferably in a range from100 Å to 5000 Å, most preferably from 500 Å to 2000 Å.

[0053] When the bonding layer 22 a is made of Rh or RhO, as describedabove, the barrier layer 22 a may be omitted.

[0054] The Au layer 22 c located at the top of the p-type pad electrode22 is the best material for wire bonding of the nitride semiconductordevice. The thickness of the Au layer is preferably in a range from 1000Å to 20000 Å, more preferably from 5000 A to 10000 A.

[0055] According to the present invention, the p-type ohmic electrode 20is made of at least one kind selected from a group consisting of Ni, Co,Fe, Ti and Cu, and Au. Ni, Co, Fe, Ti and Cu are all metal elements thatcan turn into ions having valence of 2. After forming layers from onekind selected from a group consisting of Ni, Co, Fe, Ti and Cu, and Auone on another, the layers are annealed to form an alloy, so that goodohmic contact with the p-type nitride semiconductor layer is achieved.Annealing is carried out at a temperature at which the nitridesemiconductor is not subjected to an adverse effect such as dissociationof In from InGaN that is grown before forming the ohmic electrode,preferably in a range from 400° C. to 700° C., more preferably from 500°C. to 650° C. Best ohmic characteristic of the p-type ohmic electrode 20can be achieved by selecting Ni from the group of metals described aboveand using Ni and Au. Since the alloy, that is formed by annealing thelayers of Ni and Au formed one on another, includes Ni, the beststructure may be obtained when the bonding layer 22 a of the p-type padelectrode 22 formed through partial contact with the p-type ohmicelectrode is made from Ni, which leads to higher bonding strengthbetween Ni atoms. The total thickness of the layers made of one of thegroup of metals described above or Ni and Au is preferably in a rangefrom 150 Å to 5000 Å, most preferably 1500 Å.

[0056] When an RhO layer is formed as the bonding layer 22 a, it ispreferable to form the p-type pad electrode after forming the RhO layeron the Au layer of the p-type ohmic electrode 20 and then annealing thelayers.

[0057] The heat resistance of the p-type pad electrode 22 can beimproved further by making the p-type ohmic electrode 20 in Ni—Au—Rhstructure and forming the p-type pad electrode 22 that includes the RhOlayer as the bonding layer 22 a.

[0058] Electrode characteristics of various combinations of theconstitution of the p-type ohmic electrode 20 and the constitution ofthe p-type pad electrode described above are shown in Table 1 incomparison with regard to heat resistance.

[0059] The heat resistance of the p-type pad electrode was evaluated interms of the temperature at which the ohmic characteristic changed.

[0060] Change in the ohmic characteristic was observed by forming ap-type ohmic electrode and a p-type pad electrode at a predetermineddistance from each other on a p-type gallium nitride compoundsemiconductor layer, and measuring the resistance between the twoelectrodes. TABLE 1 Temperature at which p-type ohmic p-type pad ohmiccharacteristic No. electrode electrode AuSn changed 1 Ni-Au Ni-Au No 3252 Ni-Au Ni-Au Present 275 3 Ni-Au Ni-Ti-Au No 325 4 Ni-Au Ni-Ti-AuPresent 325 5 Ni-Au Rh-Au No 325 6 Ni-Au Rh-Au Present 325 7 Ni-AuRhO-Au No 350 8 Ni-Au RhO-Au Present 350 9 Ni-Au RhO-Pt-Au No 350 10Ni-Au RhO-Pt-Au Present 350 11 Ni-Au-RhO Ni-Ti-Au No 300 12 Ni-Au-RhONi-Ti-Au Present 300 13 Ni-Au-RhO RhO-Au No 375 14 Ni-Au-RhO RhO-AuPresent 375 15 Ni-Au-RhO RhO-Pt-Au No 375 16 Ni-Au-RhO RhO-Pt-Au Present375

[0061] The thickness was set as follows for the Ni, Au and RhO layers ofthe p-type pad electrode and the p-type ohmic electrode that were usedin the test summarized in Table 1.

[0062] Ni layer thickness was set to 100 Å and Au layer thickness wasset to 1300 Å for the p-type ohmic electrode of No.1 through No.10.

[0063] Ni layer thickness was set to 100 Å, Au layer thickness was setto 660 Å and RhO layer thickness was set to 1500 Å for the p-type ohmicelectrode of No.11 through No.16.

[0064] The thickness of the layer corresponding to the bonding layer andthe barrier layer was set to 1500 Å and Au layer was set to 6000 Å forthe p-type pad electrode shown in Table 1.

[0065] Data of Nos.1 and 2 in Table 1 are shown for comparison.

[0066] Presence of AuSn is shown by whether AuSn is soldered (Present)or not (No) on the p-type pad electrode, since heat resistance oftendeteriorates after soldering of AuSn.

[0067] As described above, forming the p-type pad electrode from RhO—Auor RhO—Pt—Au makes it possible to prevent alloying with the p-type ohmicelectrode and prevent AuSn, that is usually used for the connection ofthe p-type pad electrode with other electrodes, from diffusing into thep-type ohmic electrode.

[0068] Therefore, forming the p-type pad electrode from RhO—Au orRhO—Pt—Au makes it possible to prevent service life from deterioratingdue to the p-type ohmic electrode and the p-type pad electrode andachieve the nitride semiconductor laser diode of longer service life.

[0069] In order to study the relationship between the heat resistanceand service life of the p-type ohmic electrode and the p-type padelectrode, a laser diode (LD1) having the p-type ohmic electrode and thep-type pad electrode shown as No.7 in Table 1 and a laser diode (LD2)having the p-type ohmic electrode and the p-type pad electrode shown asNo.3 in Table 1 were made and tested to determine the service life underthe conditions of continuous oscillation with 5 mW of output power at50° C. In a test after connecting by wire bonding, the LD1 showedservice life of 5934 hours in average of three samples and LD2 showedservice life of 1805 hours in average of three samples.

[0070] In a test after connecting by flip chip bonding, the LD1 showedservice life of 3346 hours in average of three samples.

[0071] The n-type pad electrode 23 comprises three layers of a bondinglayer 23 a, a barrier layer 23 b and an Au layer 23 c.

[0072] In this embodiment, the bonding layer 23 a of the n-type padelectrode 23 is made of such a material that bonds well with the secondinsulation film 31, that is formed on a part of the n-type contactlayer, and with the n-type ohmic electrode 21 and is less likely todiffuse. In case the second insulation film is made of an oxide, Ni thathas good bonding characteristic with the second insulation film 31 isthe most preferable material. Thickness is preferably in a range from100 Å to 5000 Å, most preferably from 500 Å to 2000 Å.

[0073] In this embodiment, the barrier layer 23 b of the n-type padelectrode 23 is made of such a high-melting point metal as Au atomsincluded in the top layer does not diffuse into the bonding layer or thelower layers or a nitride thereof, for example, Ti, Pt, W, Ta, Mo orTiN, and most preferably Ti. The thickness of the barrier layer 23 b ispreferably in a range from 100 Å to 5000 Å, and most preferably from 500Å to 2000 Å.

[0074] The Au layer 23 c located at the top of the n-type pad electrode23 is the material most suitable for wire bonding of the nitridesemiconductor laser device. The thickness of the Au layer is preferablyin a range from 1000 Å to 20000 Å, and more preferably from 5000 Å to10000 Å.

[0075] The constitution of the n-type pad electrode 23 may be the sameas or different from the constitution of the p-type pad electrode 22.But it is preferable to make the n-type pad electrode 23 in the sameconstitution as that of the p-type pad electrode 22, since the laserdevice manufacturing process can be simplified thereby.

[0076] According to the present invention, the n-type ohmic electrode 21may be made by forming Ti and Au layers one on another that arematerials having high ohmic contact capability with the n-type nitridesemiconductor and high bonding characteristic. In order to improve theohmic characteristic further, the layers are preferably annealed so asto form an alloy. Annealing is carried out at a temperature at which thenitride semiconductor is not subjected to an adverse effect such asdissociation of In from InGaN that is made before forming the ohmicelectrode, similarly to the case of forming the p-type ohmic electrode,preferably in a range from 400° C. to 700° C., more preferably 500° C.to 650° C. The total thickness of the Ti/Al layer for the n-type ohmicelectrode is preferably in a range from 150 Å to 10000 Å, and mostpreferably 5000 Å. Other materials that have high ohmic contactcapability and high bonding characteristic include alloys made byannealing layers of W/Al, Ti/Au, V/Al, V/Au or the like, and singleelements such as Al, Ti and W.

[0077] According to the present invention, annealing of the p-type ohmicelectrode and the n-type ohmic electrode is preferably carried out inoxygen atmosphere since good ohmic characteristic can be obtained bysupplying oxygen.

[0078] The first insulation film that covers the nitride semiconductorsuch as the p-type nitride semiconductor layer where the ridge is formedin the present invention is made of an oxide, for which ZrO₂ or the likecan be preferably used.

[0079] In this embodiment, the first insulation film 30 that covers thenitride semiconductor such as the p-type nitride semiconductor layerwhere the ridge is formed is preferably made of an oxide that can endurethe annealing temperature. The second insulation film 31 is alsopreferably made of an oxide, such as SiO₂ or TiO₂. When the secondinsulation film 31 is made of an oxide, strong bonding can be achievedbetween the bonding layers of the p-type pad electrode and the n-typepad electrode. The second insulation film can also be made of the samematerial and in the same process as the reflector film formed on theresonating end face of the laser, in which case it is preferably formedin a multi-layered film of SiO₂ and TiO₂. Specifically, two pairs ofSiO₂ film 700 Å thick and TiO₂ film 400 Å thick (SiO₂/TiO₂) may beformed one on another, so that the portion at the end face functions asa reflector film and the rest functions as a protective film.

[0080] According to the present invention, the pair of (SiO₂/TiO₂) mayalso be stacked more than two times, and the second insulation film 31that can also function as the reflector film of the laser is not limitedto the pair of (SiO₂/TiO₂), as a matter of course.

[0081] Now other constitutions according to this embodiment will bedescribed below. It needs not to say that the present invention is notlimited to the constitutions described below.

[0082] The substrate may be made of a different material such assapphire, or may be a GaN substrate that is made by a known process. Abuffer layer made of GaN is preferably formed on the substrate, whichenables it to achieve good crystallinity of nitride semiconductor to beformed on the substrate later. The buffer layer is particularlyeffective when forming the nitride semiconductor on the substrate of adifferent material. The substrate of different material refers to asubstrate made of a material other than the nitride semiconductor.

[0083] The nitride semiconductor of the present invention may be formedin any layer structure. The nitride semiconductor may be grown by avapor phase growth process such as metal organic vapor phase epitaxy(MOVPE) or hydride vapor phase epitaxy (HDCVD).

[0084] The n-type contact layer is used to form the n-type electrode, soas to improve the ohmic characteristic by doping with an n-type impuritysuch as Si. After forming the p-type layer, the p-type layer is etchedso as to expose a part of the n-type contact layer, and the n-typeelectrode is formed on the exposed n-type contact layer.

[0085] A crack prevention layer is formed on the n-type contact layer inorder to reduce the occurrence of cracks in the substrate by makingundoped constitution. The crack prevention layer may also be made ofInGaN or the like so as to have a refractive index different from thatof the n-type cladding layer, so that light emitted from the activelayer is prevented from reflecting on the substrate of differentmaterial and returning into the nitride semiconductor layer. This layermay be omitted.

[0086] The n-type cladding layer may be formed either in a single layerdoped with an n-type impurity such as Si or in a super lattice structureconsisting of an undoped layer and a layer doped with an n-type impurityformed one on another, so as to function as a layer that supplieselectrons to the active layer as well as a layer that confines carrierand light in the active layer.

[0087] The n-type optical guide layer constitutes an optical waveguidetogether with the active layer, by compensating for the reducedthickness of the active layer of multiple quantum well structure or thelike. Therefore, the n-type optical guide layer is made in such aconstitution that has a sufficient difference in refractive index fromthe n-type cladding layer and less difference in refractive index fromthe active layer that formed above. This layer may be doped with ann-type impurity or undoped, and may also be formed in a super latticestructure consisting of an undoped layer and a layer doped with ann-type impurity formed one on another.

[0088] The active layer is formed in single quantum well structure madeof InGaN or multiple quantum well structure that includes at least awell layer made of InGaN and a barrier layer. When formed in multiplequantum well structure, either one or both of the well layer and thebarrier layer may be doped with impurity. Preferably the barrier layeris doped with an impurity which decreases the threshold current. Thewell layer is formed with the thickness of 30 to 60 Å and the barrierlayer is formed to the thickness of 90 to 150 Å.

[0089] An active layer having multiple quantum well structure may startwith a barrier layer and end with a well layer, start with a barrierlayer and end with a barrier layer, start with a well layer and end witha barrier layer, or start with a well layer and end with a well layer.Preferably, the active layer comprises two to five pairs of well layerand barrier layer stacked one on another starting with a barrier layer,more preferably three pairs of well layer and barrier layer stacked oneon another, which results in decreased threshold value and longerservice life.

[0090] The p-type cap layer provided on the active layer is capable ofreplenishing positive holes that tend to be less than electrons suppliedfrom the n side to the active layer by heavily doping with p-typeimpurity such as Mg. Increasing the concentration of the p-type impurityhigher than in the p-type optical guide layer and in the p-type claddinglayer results in diffusion of the p-type impurity into the p-type layerformed on the p-type cap layer, and is preferable. Moreover, this layerhas an effect of suppressing the dissociation of In of the active layer.When this function is intended as the main objective, the layer may beleft undoped. The p-type cap layer may also be omitted.

[0091] The p-type optical guide layer that includes a p-type impuritysuch as Mg may be either intentionally doped with the p-type impurity orformed without doping, since the p-type impurity diffuses from thep-type cap layer when the p-type cap layer is doped with the p-typeimpurity. The p-type optical guide layer, that is provided for thepurpose of forming the optical guide layer similarly to the n-typeoptical guide layer, and is made in such a constitution that has asufficient difference in refractive index from the p-type cladding layerand less difference in refractive index from the active layer thatformed underneath.

[0092] The p-type cladding layer serves as a positive hole supplyinglayer for the active layer, and can be formed either in a single layerdoped with a p-type impurity such as Mg or in a super lattice structureconsisting of an undoped layer and a layer doped with a p-type impurityformed one on another.

[0093] The p-type contact layer is provided for the purpose of formingthe p-type electrode, and can have good ohmic contact with the p-typeelectrode when doped with a p-type impurity such as Mg relativelyheavily.

[0094] In the semiconductor laser diode of this embodiment having theconstitution described above, the first insulation film 30 is formed forthe purpose of forming a portion of the p-type contact layer 9 thatmakes ohmic contact with the p-type ohmic electrode 20 with a highaccuracy, and the second insulation film 31 is formed for the purpose ofprotecting the device. Since this allows it to select the best materialand configuration (thickness, etc.) for the functions of the firstinsulation film 30 and the second insulation film 31, stable laseroscillation can be achieved, short circuiting due to insufficientinsulation can be prevented and leakage current can be decreased, thusresulting in the semiconductor laser diode having high reliability.

[0095] In the semiconductor laser diode of this embodiment, each of thep-type pad electrode 22 and the n-type pad electrode 23 is constitutedfrom three layers of a bonding layer that is made of a high-meltingpoint metal or nitride thereof and bonds well with the p-type ohmicelectrode or n-type ohmic electrode and the second insulation film 31, abarrier layer that prevents diffusion of Au from the layer formedthereon and an Au layer.

[0096] With this constitution, bonding performance with the ohmicelectrode and the second insulation film 31 can be made higher, anddiffusion of Au due to heat generated by the current supplied can beprevented, deterioration of the characteristics can be prevented andreliability can be improved.

[0097] Variations

[0098] In the nitride semiconductor laser device of the embodimentdescribed above, the p-type ohmic electrode 20 is formed on the topsurface of the ridge. But the present invention is not limited to thisconstitution, and the p-type ohmic electrode 20 may also be formed so asto cover the entire ridge and extends over the p-type cladding layer 8on both sides of the ridge, as shown in FIG. 2.

[0099] Operations and effects similar to those of the embodiments can beachieved also with such a constitution.

EXAMPLES

[0100] Examples of the present invention will be described below, butthe present invention is not limited thereto.

Example 1

[0101] (Buffer Layer)

[0102] A GaN substrate obtained by a known method on sapphire with theprincipal plane lying in the C plane having diameter of 2 inches is setin a MOVPE reaction vessel, and a first buffer layer made of GaN isformed to the thickness of 200 Å by using trimethyl gallium (TMG) andammonia (NH₃). After growing the first buffer layer, a second bufferlayer made of GaN is grown to the thickness of 0.5 μm while raising thetemperature.

[0103] (n-Type Contact Layer)

[0104] An n-type contact layer made of GaN doped with Si inconcentration of 1×10¹⁸/cm³ is formed to the thickness of 4 μm by usingammonia and TMG, and silane gas used as an impurity gas.

[0105] (Crack Prevention Layer)

[0106] Then a crack prevention layer made of InGaN is formed to thethickness of 0.15 μm at a temperature of 800° C. by using TMG, TMI(trimethyl indium), and ammonia.

[0107] (n-Type Cladding Layer)

[0108] After growing layer an undoped AlGaN layer to the thickness of 25Å at a temperature of 1050° C. by using TMA (trimethyl aluminum), TMGand ammonia, supply of TMA is stopped and silane gas is supplied, and alayer made of n-type GaN doped with Si in concentration of 1×10¹⁹/cm³ isformed to the thickness of 25 Å. Super lattice structure is formed bystacking these layers, so as to form the n-type cladding layer havingsuper lattice of the total thickness 1.2 μm.

[0109] (n-Type Optical Guide Layer)

[0110] Then at a similar temperature, an n-type optical guide layer madeof undoped GaN is formed to the thickness of 750 Å by using TMG andammonia as the stock material gas.

[0111] (Active Layer)

[0112] Then by setting the temperature to 800° C., a barrier layer madeof InGaN doped with Si in a concentration of 5×10¹⁸/cm³ to the thicknessof 100 Å by using TMG and TMI and ammonia as the stock material gas andsilane gas as the impurity gas. Then, with the temperature being loweredto 820° C., the supply of silane gas is stopped and a well layer made ofundoped InGaN is formed to the thickness of 50 Å. The barrier layer andthe well layer are further stacked two times followed by the lastformation of barrier layer, thereby to form the active layer of multiplequantum well structure (MQW) having the total thickness of 550 Å.

[0113] (p-Type Cap Layer)

[0114] Then a p-type cap layer made of p-type GaN doped with Mg in aconcentration of 1×10²⁰/cm³ is formed to a thickness of 100 Å bystopping the supply of TMI and supplying Cp₂Mg.

[0115] (p-Type Optical Guide Layer)

[0116] Then with the supply of Cp₂Mg being stopped, a p-type opticalguide layer made of undoped GaN is formed to a thickness of 0.1 μm at atemperature of 1050° C. While the p-type optical guide layer is grown asan undoped layer, diffusion of Mg from the p-type cap layer increasesthe Mg concentration to 5×10¹⁶/cm³ and turns the layer to p-type.

[0117] (p-Type Cladding Layer)

[0118] Then with the supply of Cp₂Mg being stopped and TMA beingsupplied, a layer of undoped AlGaN is formed to a thickness of 25 Å at1050° C. Then the supply of TMA is stopped and Cp₂Mg is supplied, alayer of Mg-doped GaN is formed to a thickness of 25 Å with Mgconcentration of 1×10¹⁹/cm³, thereby forming the p-type cladding layerconstituted from super lattice structure of the total thickness of 0.6μm.

[0119] (p-Type Contact Layer)

[0120] Last, a p-type contact layer made of p-type GaN doped with Mg ina concentration of 1×10²⁰/cm³ is formed to the thickness of 150 Å on thep-type cladding layer.

[0121] (Formation of Ridge)

[0122] After forming the nitride semiconductor layers as describedabove, the wafer is taken out of the reaction vessel, and an SiO₂ maskis formed on a part of the p-type nitride semiconductor layer so as toexpose the n-type nitride semiconductor layer, and the surface of then-type contact layer is exposed by the RIE (reactive ion etching)process.

[0123] Then a protective film of SiO₂ is formed in a stripe pattern of1.5 μm in width on the p-type nitride semiconductor layer via a mask ofa predetermined shape on the n-type nitride semiconductor layer that hasbeen exposed. After forming the protective film, a waveguide (ridge) ina stripe pattern of 1.5 μm in width is formed by etching to near theinterface between the p-type cladding layer and the p-type optical guidelayer by RIE as shown in FIG. 1.

[0124] (First Insulation Film)

[0125] After forming the ridge, the first insulation film made of ZrO₂is formed on the surface of the p-type nitride semiconductor layer whileleaving the SiO₂ mask to remain thereon. The first insulation film mayalso be formed over the entire surface of the nitride semiconductorlayer by masking then-type ohmic electrode. After forming the firstinsulation film, the device is immersed in buffered hydrofluoric acid soas to dissolve and remove the SiO₂ from the p-type contact layer, andZrO₂ formed on the p-type contact layer (or further on the n-typecontact layer) is removed together with SiO₂by lift-off process.

[0126] (Ohmic Electrode)

[0127] Then the p-type ohmic electrode made of Ni and Au in stripeconfiguration is formed in contact with the ridge surface provided onthe p-type contact layer and the first insulation film.

[0128] Also the n-type ohmic electrode made of Ti and Al in stripeconfiguration is formed in contact with the surface of the n-typecontact layer (and in contact with the first insulation film).

[0129] After forming these layers, both the p-type and n-type ohmicelectrodes are turned into alloys by annealing in an atmosphereconsisting of oxygen and nitrogen in concentrations of proportion 80:20at 600° C., thereby to achieve good ohmic characteristic.

[0130] (Second Insulation Film)

[0131] Then the second insulation film made of SiO₂ is formed over theentire surface, and a resist is applied to the p-type ohmic electrodeand the n-type ohmic electrode except for a part thereof, and the partof the p-type ohmic electrode and the n-type ohmic electrode is exposedby dry etching.

[0132] (Pad Electrode)

[0133] After forming the second insulation film, a bonding layer made ofNi is formed to the thickness of 1000 A as the pad electrode in a singleprocess so as to cover the second insulation film provided on the p-typenitride semiconductor layer and the p-type ohmic electrode on the pside, and cover a part of the second insulation film and the n-typeohmic electrode on the n side.

[0134] A barrier layer made of Ti is formed on the bonding layer to thethickness of 1000 Å, and then an Au layer is formed to the thickness of8000 Å.

[0135] After forming the p-type pad electrode and the n-type padelectrode as described above, the nitride semiconductor is etched in amesh pattern by RIE till the sapphire substrate is exposed, so as toseparate the nitride semiconductor into chips. At this time, the laseroutput plane is etched at a position immediately before the laser outputend face so that good FFP (far field pattern) of laser beam is obtained.After etching, the sapphire is scribed along the mesh pattern where thesapphire is exposed by etching, so as to make laser chips. Separation ofthe laser chips may also be done by cleaving the GaN along M plane ofthe nitride semiconductor (the plane that corresponds to a side surfacewhen the nitride semiconductor is represented by a hexagonal prism).

[0136] In an oscillation test of the laser chip having electrodeswire-bonded thereon at the room temperature, continuous oscillation atwavelength of 405 nm with an output power of 30 mW was confirmed withthreshold of 2.0 kA/cm² at room temperature and service life of 1000hours or longer.

Example 2

[0137] A nitride semiconductor laser device was made in the same manneras in Example 1, except for forming the second insulation film fromTiO₂.

[0138] In a test of this laser chip conducted in the same manner as inExample 1, continuous oscillation at wavelength of 405 nm with an outputpower of 30 mW was confirmed with threshold of 2.0 kA/cm² at roomtemperature and service life of 1000 hours or longer.

Example 3

[0139] A nitride semiconductor laser device was made in the same manneras in Example 1, except for forming the n-type pad electrode and thep-type pad electrode from Pt.

[0140] In a test of this laser chip, continuous oscillation atwavelength of 405 nm with an output power of 30 mW was confirmed withthreshold of 2.2 kA/cm² at room temperature and service life of 1000hours or longer, that are nearly the same as those of Example 1.

Example 4

[0141] A nitride semiconductor laser device was made in the same manneras in Example 1, except for forming the bonding layers of the n-type padelectrode and the p-type pad electrode from Ti and forming the barrierlayer from Pt.

[0142] In a test of this laser chip, continuous oscillation atwavelength of 405 nm with an output power of 30 mW was confirmed withthreshold of 2.2 kA/cm² at room temperature and service life of 1000hours or longer, that are nearly the same as those of Example 1.

Example 5

[0143] A nitride semiconductor laser device was made in the same manneras in Example 1, except for forming the p-type pad electrode in theconstitution of Ni/Ti/Au and the n-type pad electrode in theconstitution of Ti/Pt/Au with bonding layer made of Ti and the barrierlayer made of Pt.

[0144] Although the manufacturing process for this nitride semiconductorlaser device is more complex than that of Example 1, continuousoscillation at wavelength of 405 nm with an output power of 30 mW wasconfirmed with threshold of 2.1 kA/cm² at room temperature and servicelife of 1000 hours or longer.

Example 6

[0145] The ohmic electrode was formed in the same manner as in Example 1with the following exception.

[0146] (Ohmic Electrode)

[0147] The p-type ohmic electrode us formed in stripe configuration fromNi and Au in contact with the ridge surface provided on the p-typecontact layer and the first insulation film.

[0148] Also the n-type ohmic electrode made of Ti and Al in stripeconfiguration is formed in contact with the surface of the n-typecontact layer (and in contact with the first insulation film).

[0149] After forming these electrodes, annealing was applied at 600° C.in an atmosphere of 100% oxygen.

[0150] The nitride semiconductor laser device made in the same manner asin Example 1, except for the annealing 100% oxygen atmosphere showedcontinuous oscillation at wavelength of 405 nm with an output power of30 mW was confirmed with threshold of 2.2 kA/cm² at room temperature andservice life of 1000 hours or longer.

INDUSTRIAL APPLICABILITY

[0151] As described in detail above, the nitride semiconductor laserdevice of the present invention has such a novel structure as the twoinsulation films are provided that enables it to precisely control thewidth of contact between a p-type ohmic electrode and a p-type contactlayer, and therefore the nitride semiconductor laser device that hasstable characteristics can be provided.

[0152] In the nitride semiconductor laser device of the presentinvention, good ohmic contact with the nitride semiconductor can beachieved with wire bonding easily carried out, and diffusion of Au intoother layers due to heat generated by the current supplied can beprevented, and therefore highly reliable nitride semiconductor laserdevice with less deterioration in the characteristics can be provided.

1. A nitride semiconductor laser device comprising: a substrate, ann-type nitride semiconductor layer on said substrate, an active layer onsaid n-type nitride semiconductor layer, a p-type nitride semiconductorlayer on said active layer, said p-type nitride semiconductor layerhaving p-type contact layer as a top layer and a ridge that includes atleast said p-type contact layer and, a p-type ohmic electrode that makesohmic contact with said p-type contact layer of said the ridge beingformed substantially parallel to a direction of resonance, wherein afirst insulation film that having an aperture for opening an uppersurface of said ridge is formed so as to cover at least a side face ofsaid ridge and a proximate region outside of said side face, whereinsaid p-type ohmic electrode is formed so as to make contact with saidp-type contact layer through said aperture, and a second insulation filmis formed on said first insulation film.
 2. The nitride semiconductorlaser device according to claim 1; wherein said second insulation filmis formed in continuation with a resonating end face so as to form alaser reflection plane on said resonating end face.
 3. The nitridesemiconductor laser device according to claims 1 or 2; wherein saidfirst insulation film and said second insulation film are made of oxidecompound.
 4. The nitride semiconductor laser device as in one of claims1-3; wherein said first insulation film is made of ZrO₂.
 5. The nitridesemiconductor laser device as in one of claims 1-4; wherein said secondinsulation film is made of TiO₂ or SiO₂.
 6. The nitride semiconductorlaser device as in one of claims 1-4; wherein said second insulationfilm is a multi-layered film made by forming TiO₂ layer and SiO₂ layerone on another.
 7. The nitride semiconductor laser device as in one ofclaims 1-6; wherein said p-type ohmic electrode is an alloy which isformed by laminating a layer of at least one selected) from a groupconsisting of Ni, Co, Fe, Ti and Cu and an Au layer and then annealingthe layers.
 8. The nitride semiconductor laser device as in one ofclaims 1-7; wherein said second insulation film is formed so as to havean aperture on said p-type ohmic electrode and a p-type pad electrode isformed so as to make contact with said p-type ohmic electrode throughsaid aperture.
 9. The nitride semiconductor laser device according toclaim 8; wherein said p-type pad electrode includes a bonding layer thatmakes contact with said p-type ohmic electrode, a barrier layer on saidbonding layer and an Au layer on said barrier layer.
 10. The nitridesemiconductor laser device according to claim 9; wherein said bondinglayer of said p-type pad electrode includes at least one selected from agroup consisting of Ni, Cu, Ru, RuO₂, Ti W, Zr, Rh and RhO.
 11. Thenitride semiconductor laser device according to claims 9 or 10; whereinsaid barrier layer of said p-type pad electrode includes at least oneselected from a group consisting of Ti, Pt, W, Ta, Mo, nitride thereofand RhO.
 12. The nitride semiconductor laser device as in one of theclaims 9-11; wherein said n-type nitride semiconductor layer includes an-type contact layer that is partially exposed and an n-type ohmicelectrode is formed on the exposed n-type contact layer and an n-typepad electrode is formed on said n-type ohmic electrode, said n-type padelectrode being made of the same material as said p-type pad electrode.13. The nitride semiconductor laser device as in one of the claims 1-6;wherein said p-type ohmic electrode is made of an alloy which is formedby laminating a layer of at least one kind selected from a groupconsisting of Ni, Co, Fe, Ti and Cu and an Au layer and then annealingthe layers, wherein said second insulation film is formed so as to havean aperture located over said p-type ohmic electrode and a p-type padelectrode is formed so as to make contact with said p-type ohmicelectrode through said aperture.
 14. The nitride semiconductor laserdevice according to claim 13; wherein said p-type pad electrode isconstituted from a bonding layer made of Rh or RhO in contact with saidp-type ohmic electrode, and an Au layer formed on said bonding layer.15. The nitride semiconductor laser device according to claim 13;wherein said p-type pad electrode is constituted from a bonding layermade of Rh or RhO in contact with said p-type ohmic electrode, a barrierlayer on said bonding layer including at least one material selectedfrom a group consisting of Ti, Pt, W, Ta, Mo and nitride thereof, and anAu layer on the barrier layer.
 16. The nitride semiconductor laserdevice according to claims 14 or 15; wherein a top layer of said p-typeohmic electrode is an RhO layer and said bonding layer is made of RhO.17. A nitride semiconductor device comprising; a p-type nitridesemiconductor layer, a p-type ohmic electrode on said p-type nitridesemiconductor layer, and a p-type pad electrode on said p-type ohmicelectrode, wherein said p-type ohmic electrode is made of an alloy bylaminating a layer of at least one selected from a group consisting ofNi, Co, Fe, Ti and Cu and an Au layer, and then annealing. wherein saidp-type pad electrode is constituted from a bonding layer made of Rh orRhO in contact with said p-type ohmic electrode, a barrier layer on saidbonding layer made of at least one material selected from a group of Ti,Pt, W, Ta, Mo and nitride thereof, and an Au layer on said barrierlayer.
 18. The nitride semiconductor laser device according to claims17; wherein said p-type a top layer of said p-type ohmic electrode is anRhO layer is included in and said bonding layer is made of RhO.
 19. Amethod for forming an electrodes of a nitride semiconductor devicecomprising; a step of forming a p-type ohmic electrode by laminating afirst layer made of at least one selected from a group consisting of Ni,Co, Fe, Ti and Cu, an Au layer and an RhO layer successively on saidp-type nitride semiconductor layer, a step of annealing said p-typeohmic electrode, a step of forming an RhO layer on said p-type ohmicelectrode that has been annealed, and a step of forming the p-type padelectrode on the p-type ohmic electrode including forming a RhO layerand forming a Au layer on said RhO layer.